So, a movie with Zod trying to iniate global genocide and Superman/US miltary trying to stop him should be light in tone, no. The movie wasn't bleak and depressing to me, I found it hopeful and uplifiting. Bad **** happens, and you overcome it and move on. Also, Superman kept having to move around because he was incapable of not helping people who needed it (Lois even mentions that that is how she found him) so I think the movie did a good job of establishing that Clark cared about life and didn't want to hurt anyone.